CD9 and cancer: Indeed, in several cancer models, VitD has been reported to sensitize tumor drug-resistant cells to chemotherapy and multi-kinase drugs acting through several mechanisms (7), mainly inducing the mesenchymal–epithelial transition (8), inhibiting pro-oncogenic pathway in cancer stem cells (30), upregulating miRNAs that reduce oncogene protein expression (8), and downregulating the expression of multi-drug-resistant protein 1 (MRP1) and multi-drug-resistant protein 5 (MRP5), efflux proteins that cause the chemotherapy and multi-kinase drugs to pump out (31, 32).